The invention relates to a method of damping a movement of a wall or a part of it, in which method the movement applied to the wall is measured and a counterforce is produced, which is opposite to the applied movement and substantially of the same size.
The invention relates further to an arrangement for damping a movement of a wall or a part of it, in which arrangement the movement applied to the wall is measured and a counterforce is produced, which is opposite to the applied movement and substantially of the same size.
Japanese Patent 07210174 discloses a noise insulating method, in which noise passing through a wall is damped. The wall acceleration caused by the noise applied to the wall is detected by sensors, and a braking force in the opposite direction is applied to the wall by an actuator to suppress the vibration detected by the sensors. However, said arrangement causes new acoustic vibration modes in the wall, which increases the stress on the wall structures.
U.S. Pat. No. 5,018,203 discloses an apparatus for attenuating noise, comprising a moving diaphragm, a movement of which, caused by noise, is measured and a counter-movement is applied to the diaphragm electrically. The intention is to make the diaphragm immovable and to produce thus a noise attenuating structure. Such an air impermeable diaphragm generates sound, which is not appropriate for attenuating vibrations.
British Patent 2 091 064 discloses a solution, in which a counter-sound is generated by a window, whereby noise is prevented from entering the building through the window. In this solution, the windows constitute an electrostatic speaker generating countersound. Said solution is complicated and inconvenient and suitable for being used in connection with flexible structures only.
The object of the present invention is to provide a method and an arrangement by which a wall movement can be damped in a fairly simple and efficient manner.
The method according to the invention is characterised in that the counterforce is produced by means of an air-permeable mass plate, which is substantially parallel with a wall surface, by controlling the strength of an electric field between the wall surface and the mass plate by means of a signal obtained from the of the wall movement in order to move the mass plate.
Further, the arrangement according to the invention is characterised in that a means for producing the counterforce is constituted by an air-permeable mass plate, which is subsequently parallel with a wall surface, and that the arrangement comprises means for controlling the strength of an electric field between the wall surface and the mass plate by means of a signal obtained from the measurement of the wall movement.
An essential idea of the invention is that a wall movement is damped by means of an air-permeable mass plate, which is parallel with a wall surface, by controlling the strength of an electric field between the wall surface and the mass plate in order to provide a force opposite to the wall-moving force. The idea of a preferred embodiment is that the mass plate is manufactured of a porous material. The idea of a second preferred embodiment is that part of the mass plate is separated to generate a signal proportional to the acceleration. The idea of a third preferred embodiment is that the wall is constituted by at least two stator plates connected to each other and a mass plate between them.
An advantage of the invention is that no new vibration points are produced on the wall when a wall movement is damped by means of a mass plate. Because the mass plate is air-permeable, it does not cause any pressure fluctuation on its different sides, and therefore, it does not generate any sound wave, but only damps the vibration of the wall by the acceleration of its mass. The porous mass plate absorbs high sounds also passively and acts as a good electrical and mechanical protection, and in addition, a resonance of the movement is prevented thanks to flow losses generated in the porous material.